Formation density measurements are typically used to calculate formation porosity. Conventional density logging (wireline or measurement while drilling) is based on the detection of attenuated gamma rays emitted from a radioactive source. After gamma rays from the source penetrate the borehole and formation, the gamma ray detectors count a fraction of the scattered gamma rays. The tool configuration usually includes the radioactive source and the dual detectors spaced at different lengths from the source. The scattering, which the gamma rays experience after emission from the source and prior to detection, is related to formation bulk density. More specifically, the number of gamma rays scattered is exponentially related to the formation electron density. Since nuclear emission from a radioactive source is random but probabilistic in occurrence, the average count rate must be taken over a period of time long enough to obtain a number of counts sufficient for a statistically accurate count rate measurement.
In measurement while drilling (MWD) tools used for making formation density measurements density tool electronics and the gamma detectors (both the short space and long space detector) may be disposed in a stabilizer blade affixed to a drill collar in a lower portion of the drill string near the drill bit. The stabilizer blade displaces drilling mud in the annulus of the borehole and places low density windows, installed radially outward of the radiation source and detectors, in contact with the earth formation. During rotary drilling, the MWD tool may typically rotate at a rate of as much as one or two revolutions per second. To account for statistics, data sampling times in the MWD tool are longer than those used with wireline density tools, and are typically in the range of about 30 seconds.
Although these measurements are taken in both wireline and MWD applications, performing porosity measurements and density measurements while drilling results in certain advantages over conventional wireline porosity and density measurements. Longer sample periods due to the slower nature of the drilling process reduce the statistical variations and uncertainty in measuring while drilling porosity and density measurements. Many of the borehole effects that perturb wireline measurements of porosity or density are reduced because the drill collar substantially fills the borehole while drilling. Also, formation effects, lithology and salinity changes under drilling conditions are comparable to or less than those for an open hole wireline measurement which may occur hours or even days after the borehole is drilled. However, in MWD applications, the washing action of drilling fluid during drilling operations can produce variations in borehole size. Increased variations in borehole diameter are called washouts. Separation or “standoff”, of the tool from the borehole wall causes measured data perturbations. The occurrence of washouts exacerbates the standoff effect.
Two basic conventional techniques are used to process dual detector count rate data. These techniques are commonly referred to as the “ratio” and “spine and rib” methods. The ratio method utilizes the ratio of detector responses to determine the parameter of interest. If the logging tool or sonde is calibrated in a reference “standard” well, and if the count rates produced by the two detectors are affected by the same proportion in non-standard environmental conditions, the ratio of count rates will tend to cancel the adverse effects of the non-standard environmental conditions. This technique is used in dual thermal neutron porosity logging. If, however, non-standard environmental conditions vary the count rates in each detector by different proportions, as when variations in borehole diameter vary the detector count rates, the spine and rib method may be more effective in determining borehole and environmental characteristics. Spine and rib analysis may be performed by plotting values obtained from the respective radiation detectors operating in the non-standard condition on a graph of values obtained from the sonde operating in known reference standard boreholes. The data obtained from the reference standard is referred to as the “spine”, whereas the effect of non-standard environmental conditions is reflected in spine-intersecting lines referred to as “ribs”. The point of intersection of a rib with the spine provides an indication of a corrected logging datum, for example, formation porosity.
Formation measurements such as the formation density are affected by tool standoff. As a result, is it necessary to correct this formation measurement. When the tool standoff gets too high, the classical spine and rib method is not enough to correct properly the density. This standoff condition exists in wireline tools but is even more severe with LWD tools where the standoff is much higher than with pad tools. During the density measurement process, it is necessary to correct the measurement in view of the affect of the tool standoff.
As mentioned, density correction is done using the apparent response of two detectors with different spacing to the source and therefore different sensitivity to standoff. By combining those two apparent densities with the spine and rib correction method, it is possible to correct the long spacing reading for the effect of the standoff This robust method works well for small standoff but is severely inadequate when the tool standoff increases. Two main reasons contribute to this inadequacy:
The rib angle or shape is mud dependent and therefore correction errors get large when standoff increases                When short spacing saturates (reading mud) the method cannot work.        
These limitations are not normally an issue with a pad tool such as a wireline tool, but with LWD tools, the standoffs encountered are much higher and the limitations of the spine and rib method can be a concern in large sections of the well.
There remains a need for a method for taking formation density measurements while drilling that corrects the measurement for the affects of substantial tool standoff.